net/dns: log why resolved does not look like it's on use [Linux]

Updates #3742
Updates #3531

Change-Id: I9fc7fa0f4bcab1cf8001ba92408c660a5b25f105
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_linux.go

@@ -95,6 +95,7 @@ func dnsMode(logf logger.Logf, env newOSConfigEnv) (ret string, err error) {
 		// try to program resolved in that case.
 		// https://github.com/tailscale/tailscale/issues/2136
 		if err := resolvedIsActuallyResolver(bs); err != nil {
+			logf("dns: resolvedIsActuallyResolver error: %v", err)
 			dbg("resolved", "not-in-use")
 			return "direct", nil
 		}
@@ -184,6 +185,7 @@ func dnsMode(logf logger.Logf, env newOSConfigEnv) (ret string, err error) {
 		// Sometimes, NetworkManager owns the configuration but points
 		// it at systemd-resolved.
 		if err := resolvedIsActuallyResolver(bs); err != nil {
+			logf("dns: resolvedIsActuallyResolver error: %v", err)
 			dbg("resolved", "not-in-use")
 			// You'd think we would use newNMManager here. However, as
 			// explained in
@@ -300,7 +302,7 @@ func resolvedIsActuallyResolver(bs []byte) error {
 	}
 	for _, ns := range cfg.Nameservers {
 		if ns != netaddr.IPv4(127, 0, 0, 53) {
-			return errors.New("resolv.conf doesn't point to systemd-resolved")
+			return fmt.Errorf("resolv.conf doesn't point to systemd-resolved; points to %v", cfg.Nameservers)
 		}
 	}
 	return nil

net/dns/manager_linux_test.go

@@ -34,8 +34,9 @@ func TestLinuxDNSMode(t *testing.T) {
 				resolvDotConf(
 					"# Managed by NetworkManager",
 					"nameserver 10.0.0.1")),
-			wantLog: "dns: [rc=nm resolved=not-in-use ret=direct]",
-			want:    "direct",
+			wantLog: "dns: resolvedIsActuallyResolver error: resolv.conf doesn't point to systemd-resolved; points to [10.0.0.1]\n" +
+				"dns: [rc=nm resolved=not-in-use ret=direct]",
+			want: "direct",
 		},
 		{
 			name:    "resolvconf_but_no_resolvconf_binary",
@@ -123,10 +124,11 @@ func TestLinuxDNSMode(t *testing.T) {
 			// alleged that it was managed by systemd-resolved, but it
 			// was actually a completely static config file pointing
 			// elsewhere.
-			name:    "allegedly_resolved_but_not_in_resolv.conf",
-			env:     env(resolvDotConf("# Managed by systemd-resolved", "nameserver 10.0.0.1")),
-			wantLog: "dns: [rc=resolved resolved=not-in-use ret=direct]",
-			want:    "direct",
+			name: "allegedly_resolved_but_not_in_resolv.conf",
+			env:  env(resolvDotConf("# Managed by systemd-resolved", "nameserver 10.0.0.1")),
+			wantLog: "dns: resolvedIsActuallyResolver error: resolv.conf doesn't point to systemd-resolved; points to [10.0.0.1]\n" +
+				"dns: [rc=resolved resolved=not-in-use ret=direct]",
+			want: "direct",
 		},
 		{
 			// We used to incorrectly decide that resolved wasn't in